1. Field of the Invention
The present invention relates to a coordinate input apparatus for optically detecting the coordinates of a pointing device.
2. Description of the Related Art
An optical coordinate input apparatus for obtaining the coordinates of a pointing device by detecting that light is blocked by the pointing device has conventionally been known. FIG. 9A shows an example of the optical coordinate input apparatus. This coordinate input apparatus shown in FIG. 9A includes sensor units 901 each including a light projecting unit and light receiving unit, and retroreflecting members 903 for retroreflecting incident light in the incident direction. A pointing device 907 existing on an input area 902 blocks light projected from the light projecting unit in the direction of the pointing device 907 and reflected toward the light receiving unit by the retroreflecting member 903. The position of the pointing device 907 is thus detected.
Japanese Patent No. 4118664 has disclosed a coordinate input apparatus in which a sensor unit is arranged behind a retroreflecting member. More specifically, a light projecting unit projects light through a gap between the retroreflecting member and an input area, and light enters a light receiving unit through the same gap. In this arrangement according to Japanese Patent No. 4118664, the sensor unit can be arranged in a position where the retroreflecting member is arranged.
In the coordinate input apparatus according to Japanese Patent No. 4118664, however, the position of a pointing device may not be detected unless the pointing device is brought into contact with the input area in the vicinity of the sensor unit. This will be explained in detail below with reference to FIG. 9B. In the arrangement of Japanese Patent No. 4118664, light emitted from the light projecting unit of the sensor unit 901 is reflected by the retroreflecting member 903, and reaches the light receiving unit of the sensor unit 901. As shown in FIG. 9B, when the pointing device 907 exists near the sensor unit 901 and is not in contact with the input area 902, the pointing device 907 does not block the light. Accordingly, the pointing device 907 is not detected.